This is because the probabilities of tsunami along the Japan Sea coast were considered to be significantly lower than those for the Pacific coast because of low seismic activity (or long recurrence intervals) in the Japan Sea area (Rikitake and Aida 1988). ( 2017) indicated, planning for tsunami disaster mitigation along the Japan Sea coast has not been as developed as for the Pacific coast. The coastal areas of the Japan Sea have experienced large tsunamis produced by earthquakes of this type: for example, the 1983 Nihonkai–Chubu earthquake tsunami (e.g., Shuto 1985). By contrast, subduction zones generating megathrust earthquakes comparable to those in the Pacific Ocean do not exist in the Japan Sea however, many active faults exist over the area. The government has therefore promoted tsunami hazard mitigation measures along the Japanese coastlines of the Pacific Ocean, where earthquake-induced tsunamis reoccur at moderate intervals and for which a great wealth of such observation data exist. 1983), and deposits for paleo tsunamis were also found (e.g., Fujino et al. There are records and survey results of recent tsunami inundation and damage in the coastal areas (e.g., Hatori et al. These earthquakes generated large tsunamis that struck the coastal areas. Historical documents have indicated that large earthquakes repeatedly occurred with intervals of 100–200 years over a long term (Thatcher 1984). Nankai Trough earthquakes are typical examples of repeatedly occurring earthquakes. Earthquakes occurring in subduction zones have been considered to occur repeatedly dominated by several factors such as the geological structure and the convergence rate.
2013) and Nankai Trough (e.g., Tanioka and Satake 2001). The regions bordering the Pacific Ocean have typically been struck by tsunamis generated by earthquakes in subduction zones, such as the Japan Trench (e.g., Satake et al. The coastal areas at risk from near-field tsunamis span the coastlines along the Pacific Ocean and the Japan Sea, covering metropolises and major ports. Japanese coastal areas, including urbanized and densely populated areas, are prone to severe damage from tsunami inundation (e.g., Woessner and Farahani 2020). Based on these results, the enhancement of existing infrastructure such as breakwaters and river levees should be the priority measure within tsunami risk-mitigation strategies these, combined with natural resources such as dunes, can effectively mitigate tsunami disasters. These findings were common to the coastal areas, and could be generally applicable to tsunami inundation for all coastal areas along the Japan Sea. Additionally, even the areas protected by the dunes might be exposed to an inundation risk in which the tsunamis invade a river channel, causing it to overflow. However, the presence of the breakwaters also increases the tsunami heights in the areas immediately beyond the ports.
According to the simulated results, the dunes and port breakwaters contribute significantly to decreasing the total amount of tsunami inundation for the areas behind them. Natural sand dunes, ports with man-made breakwaters, and rivers with levees are typical topographical features found in urbanized and densely populated areas along the Japan Sea coastline. To demonstrate the effectiveness of the estimated inundation characteristics, tsunami inundation was simulated for each coastal area under varying tsunami source conditions (including excessive tsunami magnitude) with a high resolution of the topography and infrastructure. Comprehensive simulations for tsunami propagation and inundation were performed for coastal areas in Akita City, Sakata City, and Niigata City based on earthquake and tsunami source scenarios developed by the Japanese Government for the Japan Sea area. Based on tsunami simulations for representative coastal areas, we aimed to identify common vulnerabilities, which included those suggested by previous studies and local governments, to develop disaster mitigation strategies. This study estimated the tsunami inundation characteristics for three major Japanese coastal cities along the Japan Sea. For Japanese coastal communities along the Japan Sea, where the risk of earthquake-induced tsunamis is deemed lower than that along the Pacific Ocean, tsunami disaster mitigation strategies have not been sufficiently developed.
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